How to force compositor nodes to execute?
So, it turns out there is no easy way (if any) to trigger either compositing or file-save in both the GUI and the Python API. Or at least not without re-rendering the image.

Based on this anwser I made an extension that does the following:


Make a new dummy render layer
Disable all scene layers
Render the dummy layer (and only that layer)


As a byproduct it will re-composite all your render layers and thus re-trigger the compositing of your image.

Afterwards it will delete the dummy render layer.

To make it easy I made it so that you can search for "recompose" by hitting space or hitting the "Recompose" button in the Node Editor when you're inside the Compositing Node Tree.

You can find the addon here: http://cloud.webvi.be/index.php/s/NYbHNlrVX4ZikXn